Sh2-117 - Pelican Nebula
IC5070 - Click here for full resolution
The Pelican Nebula (IC 5070) is an H II region associated with the North America Nebula (NGC7000) in the constellation Cygnus. The Pelican Nebula is located nearby first magnitude star Deneb, and is divided from its more prominent neighbour, the North America Nebula, by a foreground molecular cloud filled with dark dust. Both are part of the larger H II region of Westerhout 40. The Pelican is much studied because it has a particularly active mix of star formation and evolving gas clouds. The light from young energetic stars is slowly transforming cold gas to hot and causing an ionization front gradually to advance outward.
source: Wikipedia
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IC5070 and NGC7000
Sh2-117, Pelican Nebula, North American Nebula
Emission Nebula
Cygnus
20h 51m 00.0s
+44º 22’ 00”
28 Aug
81º S
Conditions
The images were taken on three nights in early September 2023 near Duns, Scotland. This is a travel location with typically very dark skies (SQM >21.0 mag/arcsec²). Since the moon was out (50-80% illumination) at around 80º angular distance, SQM values were significantly lower. That was why a narrowband target was shot. Humidity was quite high and throughout the sessions, quite some frames were discarded because of clouds and/or mnist.
Equipment
Since the images were taken during traveling, the only rig available was the travel rig, based on the Takahashi FS-60CB. After the first light in July of this rig, this was the first set of serious imaging nights with it. All went fairly smoothly, with good polar alignment, guiding at around 0.4” RMS and generally trouble-free equipment. One upgrade was applied to the system and that was the Antlia ALP-T dual-band (Ha, OIII) narrowband filter. The 5nm bandpasses of this filter are somewhat smaller than the 7nm bandpass of the previously used Optolong L-Extreme. Also the Antlia is supposed to be halo-free.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi FS-60CB, 1.04x multi-flattener (370mm @ f/6.2), PrimaluceLab Esatto 2”
Rainbow Astro RST-135E, Gitzo GT5533S tripod
QHY268C, cooled to -15 ºC
Antlia ALP-T, 2” mounted, Baader filter drawer
Askar FMA180 with reducer (180mm @ f/4.5), ZWO ASI290MM
Fitlet2, Linux Mint 20.04, Pegasus Powerbox Advance
KStars/Ekos 3.6.5, INDI Library 2.0.2, PixInsight 1.8.9-2
Imaging
The target was only visible until around 03:00h each night, after which it would disappear behind a house blocking the horizon. And with it getting dark still pretty late this time of year, there was not a whole lot observation time each night. But before dawn each night there was a second opportunity for imaging, as comet C/2023 Pa (Nishimura) would become visible from around 04:00h. The images were taken with an OSC camera with the Antlia ALP-T dual-band Ha/OIII filter. Narrowband images are typically taken in the High Conversion Gain mode of the camera, which for the QHY268c is 26. With a lot of discarded images due to clouds and mist, the number of frames that made it to the final image was 75, resulting in a total exposure of just over 6h.
Resolution
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
12,504 × 8,352 px (104 MP)
370 mm @ f/6.2
3.76 µm
1.045 arcsec/px (Drizzle 2x)
3° 37' 52.2" x 2° 25' 31.5"
169.1 degrees
RA: 20h 55m 23.460s
Dec: +44° 10’ 49.46”
Processing
A total of 75 good frames were calibrated (25 Flats, 50 Darks and 50 Flat-Darks), using the WBPP script in PixInsight. The native pixel scale of this rig is somewhat under sampled at 2 arcsex/px. Applying a drizzle factor of 2 (drop shrink 1.0) reduced significantly the blockiness of the stars, but also resulted in an overall image of over 100MP.
Processing of the stars was performed separate from the nebula. After calibration, a clone was made for the Stars only image. First colorcalibration was performed using SPCC, and then a simple and modest HT stretch was applied. By not stretching the image too much, the star size was nicely contained. Alternative approaches using BlurXTerminator gave tighter stars, but often with some artefacts. After stretching the Stars were isolated using StarXTerminator.
The nebula image was a bit more complex. The processing started with a gradient removal using DynamicBackgroundExtraction. It was difficult to find some background points as there was nebulosity across the frame. The stars were now removed using StarXTerminator to yield a ‘Nebula Only’ image. The next step was the tricky part of the whole processing, and that was the intention to create a typical Hubble-style palette with the blue, yellow and amber colours, based on just dual-band data. The tool that is designed for this is the NarrowbandNormalization process. There was quite a bit of experimentation necessary, but the final settings applied were as follows:
The new NarrowbandNormalization process allowed the creation of a traditional Hubble palette image with yellow, amber and blue, out of dual band narrowband data captured with an OSC camera.
After the desired colour palette was obtained, the image was stretched using the GeneralysedHyperbolicStretch process. This allowed some of the fainter areas of the nebula to come out well. In the linear stage, colours were further fine-tuned, using several colour masks. This included enhancing saturation of the blue colours to have the center of these nebulas to stand out better. Also the yellow colours were coloured a bit more amber. After the colours were adjusted to my liking, noise was reduced using NoiseXTerminator in a modest setting of 0.6. The last step was to increase the sharpness using the MultiscaleLinearTransform process. This completed the processing of the nebula-only image.
PixelMath was then used to bring the nebula-only and the stars-only image together into the final image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
